In reprography, it is known to use halftone color separations as copy masters for making offset or letterpress plates. Before the printing plates are made, the color separations are proofed by color proofing methods to determine whether the subsequent printing results will be a tonally correct reproduction of the original.
Such color proofing methods use photosensitive recording elements that produce images from differences in the tackiness of the exposed and unexposed areas of the photosensitive layer.
For example, German Patent 12 10 321, and U.S. Pat. Nos. 3,620,726; 3,582,327; 3,649,268; 4,356,253; 4,892,802; and 4,948,704, disclose a reproduction process in which a tacky, photopolymerizable recording element, comprising a support and a photopolymerizable layer containing at least one addition- polymerizable monomer and a photopolymerization initiator, is hardened by imagewise exposure, whereby the exposed image areas lose their tackiness. The latent image is then made visible by the application of suitable toners that adhere only to the unexposed, tacky image areas and that can be removed from the non-tacky image areas. This process yields positive, optionally color images of the original, resembling in appearance the images that would be produced by using printing inks.
Negative images of the original are obtained if, for example, the photosensitive elements are used as described in U.S. Pat. Nos. 4,243,741; 4,346,162; and 4,604,340. The photosensitive components in the recording elements are either a dihydropyridine compound or a system of a dihydropyridine compound and a hexaaryl bisimidazole compound.
Toners comprising predominantly finely divided powders are applied by dusting the imagewise-exposed surface with the toner, as described, for example, in U.S. Pat. Nos. 4,215,193 and 3,909,282. Alternatively, the toner can be loosely adhered on a special support and transferred by bringing the support into contact with the imagewise-exposed layer. Such supported pigment layers are described in U.S. Pat. No 4,806,451 and DE-C 39 41 493.
Photosensitive recording elements usually comprise a support, a photosensitive layer, and a cover film. The cover film is stripped off and the element is laminated on an image receptor before imagewise exposure. Depending on the element, the support can be stripped off before or after exposure.
A repeatedly recurring problem in this process is that the cover film cannot be stripped off satisfactorily before lamination on the image receptor, because the cover film adheres too firmly to the photosensitive layer. In this case, the photosensitive layer can separate partially from the support, or parts of the layer can be torn off by the cover film. Such damaged layers can, of course, not be used for the proofing process.
The equilibrium of the photosensitive layer's adhesion between the cover film and the support can indeed be somewhat improved by adding aliphatic polyesters with molecular weights between 1,500 and 40,000, as described in U.S. Pat. No. 4,326,010. However, other difficulties exist with photosensitive recording elements thus prepared. After these photosensitive recording elements are prepared, two to ten weeks are required to reach the adhesion equilibrium. This leads to undesirable storage, delayed shipment, and additional quality control. Furthermore, the polyester concentrations needed in the coating solution for preparing the photosensitive recording elements are very highly dependent on the coating thickness of the photosensitive recording element. Consequently, it is impossible to prepare photosensitive recording elements with various coating thicknesses between 2 .mu.m and 20 .mu.m from one standard coating solution. Making new coating solutions for each product change increases cost.
Moreover, adding aliphatic polyesters with molecular weights between 1,500 and 40,000, as described in U.S. Pat. No. 4,326,010, limits the selection of cover films. Polypropylene films from different manufacturers or different manufacturing processes show different adhesive properties with respect to the tonable, photosensitive layer.
Another disadvantage of the photosensitive recording element described in U.S. Pat. No. 4,326,010 is the marked temperature dependence of the adhesion equilibrium. Even at a temperature of 45.degree. C., the tonable, photosensitive recording element has lower adhesion with respect to the support, and this results in partial areas of the photosensitive layer being torn off or partial separation of the photosensitive layer from the support. Of course, such damaged layers cannot be used in color proofing processes.
Therefore, a problem addressed in this invention is the elimination of the above-cited difficulties in the color proofing process. Another problem addressed is the simplification of the process in which appropriate photosensitive recording elements are prepared.
A necessary condition is that the adhesion equilibrium of a tonable, photosensitive composition with respect to the cover film and the support, be such that (a) the cover film can be selectively stripped off first, and the support remains strippable, (b) the recording layer is not damaged in both cases, (c) the adhesion of the recording layer to an image receptor or to another recording layer is not affected, and (d) flawless toner application is ensured on the imagewise-exposed layer. These characteristics must be assured within a temperature range of 15.degree. C.-60.degree. C.
One aspect of the problem involved in this invention is to prepare a tonable, photosensitive composition that can yield various coating thicknesses in the range of 2 .mu.m to 20 .mu.m from one standard formulation.
Another aspect of the problem involved in the invention is to prepare a tonable, photosensitive composition that requires less than a week to reach adhesion equilibrium and thus can be delivered directly after preparation.
Another aspect of the problem involved in the invention is to prepare a tonable, photosensitive composition that can use polypropylene films from different manufacturing methods as the cover film.